A transistor formed over a flat plate such as a glass substrate, which is typically used in a liquid crystal display device, is generally formed using a semiconductor material such as amorphous silicon or polycrystalline silicon. A transistor manufactured using amorphous silicon has low field effect mobility, but can be formed over a larger glass substrate. In contrast, a transistor manufactured using polycrystalline silicon has high field effect mobility, but needs a crystallization step such as laser annealing and is not always suitable for a larger glass substrate.
Thus, a technique in which a transistor is manufactured using an oxide semiconductor as a semiconductor material and applied to an electronic device or an optical device has attracted attention. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a transistor is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and such a transistor is used as a switching element or the like of an image display device.
A transistor in which a channel formation region (also referred to as a channel region) is provided in an oxide semiconductor can have higher field effect mobility than a transistor using amorphous silicon. An oxide semiconductor film can be formed by a sputtering method or the like at a relatively low temperature. Its manufacturing process is easier than that of a transistor using polycrystalline silicon.
Transistors which are formed using such an oxide semiconductor over a glass substrate, a plastic substrate, or the like are expected to be applied to display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and electronic paper.